Vehicle steering system and manufacturing method for vehicle steering system

ABSTRACT

Provided is a vehicle steering system that is able to restrict a travel amount of a steered shaft in a housing, that is small and that has an excellent strength. The vehicle steering system is a steer-by-wire vehicle steering system that converts rotative power of electric motors into a movement of a steered shaft in an axial direction via a ball screw mechanism. One of contact portions at respective ends of a screw shaft at an intermediate part of the steered shaft passes through a rotor based on a moving direction of the steered shaft so as to contact a corresponding one of the corresponding stoppers thereby restricting a travel amount of the steered shaft. The stoppers are made of a material different from that of the housing and having a high strength.

TECHNICAL FIELD

The present invention relates to a vehicle steering system and amanufacturing method for a vehicle steering system.

BACKGROUND ART

In a vehicle steering system, a travel amount of a steered shaft isusually restricted by bringing a joint, which connects the steered shaftand a tie rod to each other, into contact with a rack stopper attachedto an end portion of a housing, from outside the housing. PatentDocument 1 discloses such a vehicle steering system.

Further, Patent Document 2 discloses a hydraulic power steering systemin which a rack stopper is made of a synthetic resin member, in which ametal cored bar is inserted, to suppress noise of collision between theaforementioned joint and the rack stopper.

Further, Patent Document 3 discloses a steering rack rattling noiseprevention device that prevents occurrence of rattling noise by bringinga rack end plate into contact with an elastic member attached to an endportion of a housing from outside the housing.

Further, Patent Document 4 and Patent Document 5 each disclose anelectric power steering system in which an axial travel amount of asteered shaft is restricted by bringing part of the steered shaft intocontact with a stopper portion formed in a housing itself from insidethe housing.

Further, Patent Document 6 discloses a by-wire vehicle steering systemincluding a pair of electric motors opposed to each other in an axialdirection of a steered shaft and surrounding the steered shaftconcentrically.

Further, Patent Document 7 discloses a steering system in which a ballnut is arranged radially inward of a rotor of an electric motor.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Application Publication No. 11-222146

Patent Document 2: Japanese Utility Model Application Publication No.6-037060

Patent Document 3: Japanese Utility Model Application Publication No.54-181424

Patent Document 4: Japanese Patent Application Publication No.2000-62630

Patent Document 5: Japanese Patent Application Publication No. 6-144283

Patent Document 6: U.S. Pat. No. 4,221,656

Patent Document 7: Japanese Patent Application Publication No.2001-80530

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In a case where a rear wheel is used as a steered wheel in a loadingvehicle such as a forklift, for example, the maximum steered angle isapproximately 80°, which is very large. For this reason, in PatentDocuments 1, 2, 3, it is very difficult to lay out, outside a housing, arestriction mechanism that restricts a travel amount of a steered shaft.

Further, in a case where a restriction mechanism is laid out inside ahousing, like Patent Documents 4, 5, an increase in size of the housingshould be avoided.

Further, if a stopper is provided in a housing itself, it is necessaryto ensure a sufficient strength as a housing so that the housing is beable to endure even if a steered wheel which is steered at the maximumsteering angle, for example, collides with a curb stone or the like.

On the other hand, if a stopper provided as a member different from atubular housing is mounted in a deeper side of the housing, it isexpected that working performance of assembly would worsen.

One object of the present invention is to provide a vehicle steeringsystem that is able to restrict a travel amount of a steered shaft in ahousing, that is small, and that has an excellent strength.

Means for Solving the Problem

One feature of the present invention is as follows: in a steer-by-wirevehicle steering system (1) that converts rotative power of electricmotors (21, 22) into a movement of a steered shaft (6) in an axialdirection (X1) via a ball screw mechanism (23), the vehicle steeringsystem including: a threaded shaft (32) provided at an intermediate partof the steered shaft; a ball nut (33) screwed to the threaded shaft viaballs (34) and rotatable together with a rotor (26) of the electricmotors; a tubular housing (5) that houses the electric motors and theball nut and through which the steered shaft is passed; and first andsecond stoppers (47, 48, 470, 480, 147, 148) arranged at first andsecond end portions (51, 52) of the housing, respectively, the steeredshaft has first and second contact portions (45, 46) locatedrespectively at first and second end portions of the threaded shaft,radial gaps (S1, S2) are formed between outer peripheries of the contactportions and an inner periphery of the rotor so that each of the contactportions is allowed to be inserted into the rotor, and when the steeredshaft moves in the axial direction, one of the contact portions, whichis on a moving direction side and passed through the rotor, contacts acorresponding one of the stoppers, so that a travel amount of thesteered shaft in the axial direction is restricted.

Effects of the Invention

According to the present invention, based on a moving direction of asteered shaft, corresponding contact portion of the steered shaft passesthrough a rotor so as to contact a corresponding one of stoppers,thereby restricting a travel amount of the steered shaft. Because thestoppers are formed of members different from the housing, it ispossible to form the stoppers from a material having a strength higherthan that of the housing, thereby making it possible to improve astrength of a vehicle steering system. Further, first and second contactportions housed in the rotor are formed respectively at first and secondend portions of a threaded shaft provided at an intermediate part of thesteered shaft. Accordingly, it is possible to restrict the travel amountof the steered shaft within the housing, while reducing the size in anaxial direction and a radial direction. Further, the stoppers do not runon the threaded shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating the schematic configuration of avehicle steering system according to an embodiment of the presentinvention.

FIG. 2 is a schematic sectional view of a steered shaft and a mechanismfor driving the steered shaft.

FIG. 3A is a sectional view of main portions of the vehicle steeringsystem, illustrating a state in which a travel amount of the steeredshaft is restricted based on a moving direction of the steered shaft.

FIG. 3B is a sectional view of main portions of the vehicle steeringsystem, illustrating a state in which a travel amount of the steeredshaft is restricted based on a moving direction of the steered shaft.

FIG. 4 is a schematic view illustrating the schematic configuration in acase where the steered shaft is moved axially by use of an electricmotor of the vehicle steering system in a fully-fitting step.

FIG. 5A is a schematic view illustrating a half-fitting step of amanufacturing method for a vehicle steering system.

FIG. 5B is a schematic view illustrating a fully-fitting step of themanufacturing method for a vehicle steering system.

FIG. 5C is a schematic view illustrating a fully-fitting step of themanufacturing method for a vehicle steering system.

FIG. 6 is a flowchart illustrating a control flow in the fully-fittingstep.

FIG. 7A is a sectional view of main portions of a vehicle steeringsystem according to another embodiment of the present invention.

FIG. 7B is a sectional view of main portions of a vehicle steeringsystem according to the other embodiment of the present invention.

FIG. 8A is a sectional view of main portions of a vehicle steeringsystem according to further another embodiment of the present invention.

FIG. 8B is a sectional view of main portions of a vehicle steeringsystem according to the further another embodiment of the presentinvention.

FIG. 9 is a schematic sectional view schematically illustrating afully-fitting step of axially moving a steered shaft by use of anexternal actuator in a manufacturing method for a vehicle steeringsystem according to another embodiment of the present invention.

FIG. 10 is a flowchart illustrating a control flow in the fully-fittingstep in FIG. 9.

FIG. 11 is a schematic sectional view of a steered shaft and a mechanismfor driving the steered shaft according to further another embodiment ofthe present invention.

MODES FOR CARRYING OUT THE INVENTION

FIG. 1 is a schematic view illustrating the schematic configuration of avehicle steering system according to an embodiment of the presentinvention. With reference to FIG. 1, the vehicle steering system 1constitutes a so-called steer-by-wire system in which a mechanicalcoupling between a steering member 2 such as a steering wheel andsteered wheels 3 is eliminated.

An operation of a steering actuator 4 driven in response to a rotationaloperation of the steering member 2 is converted into a linear motion, ina vehicle-width direction, of a steered shaft 6 supported by a housing5, and the linear motion of the steered shaft 6 is converted intosteered motions of the left and right steered wheels 3 for steering.Thus, steering is achieved.

A driving force (a rotational force of an output shaft) of the steeringactuator 4 is converted into a linear motion of the steered shaft 6 inan axial direction X1 (a vehicle-width direction) by a motion conversionmechanism (e.g., a ball screw mechanism) provided in association withthe steered shaft 6. The linear motion of the steered shaft 6 istransmitted to tie rods 7 provided so as to project from respective endsof the steered shaft 6, thereby causing knuckle arms 8 to pivot. Thus,steering of the steered wheels 3 supported by the knuckle arms 8 isachieved.

A steering mechanism 100 for steering the steered wheels 3 isconstituted by the steered shaft 6, the tie rods 7, the knuckle arms 8,and the like. The housing 5 that supports the steered shaft 6 is fixedto a vehicle body via a bracket (not shown), or the like.

The steering member 2 is connected to a rotary shaft 9 rotatablysupported on the vehicle body. A reaction force actuator 10 for applyingan operation reaction force to the steering member 2 is attached to therotary shaft 9. The reaction force actuator 10 includes an electricmotor, such as a brushless motor, having an output shaft formedintegrally with the rotary shaft 9.

An elastic member 11 constituted by a spiral spring or the like, forexample, is connected between the vehicle body and an end portion on theopposite side of the rotary shaft 9 from the steering member 2. When thereaction force actuator 10 applies no torque to the steering member 2,the elastic member 11 causes the steering member 2 to return to astraight steering position using an elastic force thereof.

A steering angle sensor 12 for detecting a steering angle θh of thesteering member 2 is provided in association with the rotary shaft 9 todetect an operation input value of the steering member 2. A torquesensor 13 for detecting a steering torque T applied to the steeringmember 2 is provided on the rotary shaft 9. In the meantime, a steeredangle sensor 14 for detecting a steered angle δW (tire angle) of thesteered wheels 3 is provided in association with the steered shaft 6.

In addition to these sensors, the following sensors are provided: avehicle speed sensor 15 that detects a vehicle speed V, a verticalacceleration sensor 16 serving as a rough road condition detectingsensor that detects a vertical acceleration GZ of a vehicle body 60, alateral acceleration sensor 17 that detects a lateral acceleration Gy ofa vehicle, and a yaw rate sensor 18 that detects a yaw rate γ of thevehicle.

Detection signals from the sensors 12 to 18 are input into a controldevice 19 serving as vehicle control means constituted by an electroniccontrol unit (ECU) including a microcomputer.

The control device 19 sets a target steered angle based on a steeringangle θh detected by the steering angle sensor 12, and a vehicle speed Vdetected by the vehicle speed sensor 15. The control device 19 performsdriving control (steering control) on the steering actuator 4 via adriving circuit 20A based on a deviation between the target steeredangle and a steered angle δW detected by the steered angle sensor 14.

In the meantime, the control device 19 performs driving control(reaction force control) on the reaction force actuator 10 via a drivingcircuit 20B based on the detection signals output from the sensors 12 to18 so that an appropriate reaction force in a direction opposite to asteering direction of the steering member 2 is generated.

With reference to FIG. 2, an intermediate part of the steered shaft 6 isinserted within the tubular housing 5. First and second electric motors21, 22 constituting the steering actuator 4, and a ball screw mechanism23 serving as a motion conversion mechanism that converts rotationsoutput from the electric motors 21, 22 into an axial movement of thesteered shaft 6 are arranged between an inner periphery 5 a of thehousing 5 and the steered shaft 6 inserted within the housing 5.

The first electric motor 21 and the second electric motor 22 arearranged next to each other in the axial direction X1 within the housing5. The first electric motor 21 includes a first stator 24 fixed to theinner periphery 5 a of the housing 5, and the second electric motor 22includes a second stator 25 fixed to the inner periphery 5 a of thehousing 5. The first electric motor 21 and the second electric motor 22have a common tubular rotor 26 surrounding the periphery of the steeredshaft 6.

The rotor 26 includes a tubular rotor core 27 surrounding the peripheryof the steered shaft 6, and first and second permanent magnets 28, 29fitted to an outer periphery 27 a of the rotor core 27 so as to berotatable together with the rotor core 27. The first permanent magnet 28and the second permanent magnet 29 are arranged next to each other inthe axial direction X1. The first permanent magnet 28 faces the firststator 24, and the second permanent magnet 29 faces the second stator25.

The housing 5 has a first end portion 51 and a second end portion 52. Afirst end portion 271 of the rotor core 27 is supported rotatably by afirst bearing 30 supported by the first end portion 51 of the housing 5.Further, a second end portion 272 of the rotor core 27 is supportedrotatably by a second bearing 31 supported by the second end portion 52of the housing 5. The first and second bearings 30, 31 are constitutedby angular contact ball bearings having contact angles reverse to eachother.

An axial movement of each of outer rings of the first bearing 30 and thesecond bearing 31 relative to the housing 5 is restricted, and an axialmovement of each of inner rings of the first bearing 30 and the secondbearing 31 relative to the rotor core 27 is restricted. Accordingly, anaxial movement of the rotor core 27 relative to the housing 5 isrestricted.

The ball screw mechanism 23 includes a threaded shaft 32 formed at anintermediate part of the steered shaft 6 in the axial direction X1, aball nut 33 that surrounds the periphery of the threaded shaft 32 androtates together with the rotor core 27, and a plurality of balls 34provided in a row. The balls 34 are interposed between a spiral threadgroove 35 (an internal thread groove) formed in an inner periphery ofthe ball nut 33 and a spiral thread groove 36 (an external threadgroove) formed in an outer periphery of the threaded shaft 32.

The ball nut 33 is fitted to an inner periphery 27 b of the rotor core27 so as to be rotatable together with the ball nut 33. Further, theball nut 33 is fitted in a recessed portion 27 c formed in the innerperiphery 27 b of the rotor core 27, so that an axial relative movementbetween the ball nut 33 and the rotor core 27 is restricted. In themeantime, as described above, the axial movement of the rotor core 27relative to the housing 5 is restricted via the first and second bearing30, 31. Thus, the axial movement of the ball nut 33 relative to thehousing 5 is restricted.

The housing 5 is formed by combining a first housing 37 and a secondhousing 38 together. More specifically, a first annular flange 39 formedon the first housing 37 and a second annular flange 40 formed on thesecond housing 38 butt against one another. The first and second annularflanges 39, 40 are fastened to each other by use of a fastening screwthread 41, so that the first housing 37 and the second housing 38 areconnected to each other. Due to the fastening by the fastening screwthread 41, preloads are applied from the first and second housings 37,38 to the first and second bearings 30, 31, which are angular contactball bearings.

A rotation angle sensor 42, such as a resolver, which detects a rotationangle of the rotor 26 is arranged in the housing 5. More specifically,the rotation angle sensor 42 includes a sensor stator 43 fixed to theinner periphery 5 a of the housing 5, and a sensor rotor 44 connected tothe outer periphery (the outer periphery 27 a of the rotor core 27) ofthe rotor 26 so as to be rotatable together with the rotor 26.

The threaded shaft 32 is provided at the intermediate part of thesteered shaft 6 in the axial direction X1. The steered shaft 6 has afirst contact portion 45 that is adjacent to a first end portion 321 ofthe threaded shaft 32, and has a second contact portion 46 that isadjacent to a second end portion 322 of the threaded shaft 32. The firstand second contact portions 45, 46 are formed from the same material asthat of the steered shaft 6, and formed integrally with the steeredshaft 6. Each of the contact portions 45, 46 has an outside diameterthat is substantially the same as an outside diameter of a thread partof the threaded shaft 32.

A radial gap S1 is formed between an outer periphery 45 a of the firstcontact portion 45 and an inner periphery of the rotor 26 (correspondingto the inner periphery 27 b of the rotor core 27). Further, a radial gapS2 is formed between an outer periphery 46 a of the second contactportion 46 and the inner periphery of the rotor 26 (corresponding to theinner periphery 27 b of the rotor core 27). Thus, each of the contactportions 45, 46 is able to move smoothly within the rotor core 27 inaccordance with movement of the steered shaft 6 in the axial directionX1.

In the meantime, within the housing 5, a first stopper 47 is attached toa fitting surface 53 formed at the first end portion 51 of the housing5. Further, within the housing 5, a second stopper 48 is attached to afitting surface 54 formed at the second end portion 52 of the housing 5.The first and second stoppers 47, 48 are made of a material having astrength higher than that of iron casting, for example, whichconstitutes the housing 5. The first and second stoppers 47, 48 are madeof carbon steel e.g., S45C, having a high strength.

As illustrated in FIG. 3A, which is a magnified view, the fittingsurface 53 of the first end portion 51 of the housing 5 has a loose fitportion 53 a to which an outer periphery 47 a of the first stopper 47 isfitted loosely, and a tight fit portion 53 b to which the outerperiphery 47 a of the first stopper 47 is fitted tightly. The tight fitportion 53 b is located at a deeper side (a left side in the drawing) ofa fitting stroke for the first stopper 47 than the loose fit portion 53a. Further, the first end portion 51 of the housing 5 has a receivingportion 55 that receives the first stopper 47 in the axial direction X1.

As illustrated in FIG. 3B, which is a magnified view, the fittingsurface 54 of the second end portion 52 of the housing 5 has a loose fitportion 54 a to which an outer periphery 48 a of the second stopper 48is fitted loosely, and a tight fit portion 54 b to which the outerperiphery 48 a of the second stopper 48 is fitted tightly. The tight fitportion 54 b is located at a deeper side (a right side in the drawing)of a fitting stroke for the second stopper 48 than the loose fit portion54 a. Further, the second end portion 52 of the housing 5 has areceiving portion 56 that receives the second stopper 48 in the axialdirection X1.

When the steered shaft 6 moves to the left side (the left side in FIG.2) in the axial direction X1, an end face 45 b, serving as a contactsurface, of the first contact portion 45, which is a contact portion onthe moving direction side and passed through the rotor core 27 of therotor 26, contacts an end face 47 b of the first stopper 47 serving as acontact surface, as illustrated in FIG. 3A, so that a travel amount ofthe steered shaft 6 in the axial direction X1 is restricted. The endfaces 45 b, 47 b serving as the contact surfaces are surfaces that areperpendicular to the axial direction X1.

Further, when the steered shaft 6 moves to the right side (the rightside in FIG. 2) in the axial direction X1, an end face 46 b, serving asa contact surface, of the second contact portion 46, which is a contactportion on the moving direction side and passed through the rotor core27 of the rotor 26, contacts an end face 48 b of the second stopper 48serving as a contact surface, as illustrated in FIG. 3B, so that thetravel amount of the steered shaft 6 in the axial direction X1 isrestricted. The end faces 46 b, 48 b serving as the contact surfaces aresurfaces that are perpendicular to the axial direction X1.

The housing 5 is formed by combining the first and second housings 37,38 together. Accordingly, it is necessary to insert the first stopper 47deeply into the tubular first housing 37 to attach the first stopper 47to the first housing 37. Similarly, it is necessary to insert the secondstopper 48 deeply into the tubular second housing 38 to attach thesecond stopper 48 to the second housing 38. Therefore, the work at adeeper portion is difficult, which may cause a possibility that assemblyperformance would worsen.

Therefore, as illustrated in FIG. 4, in a state where the stoppers 47,48 are temporarily fitted to the corresponding fitting surfaces 53, 54(more specifically, in a state where the stoppers 47, 48 are fitted onlyto the loose fit portions 53 a, 54 a), the steered shaft 6 is caused toreciprocate in the axial direction X1 to its stroke ends by the electricmotors 21, 22, thereby causing the stoppers 47, 48 to be fully fitted tothe corresponding fitting surfaces 53, 54 by the corresponding contactportions 45, 46 of the steered shaft 6. The timing of completion of afully-fitting step can be set as a timing that is reached after apredetermined time has elapsed from the timing which is detected basedon an output from the rotation angle sensor 42 and at which rotation ofthe rotor 26 of the electric motors 21, 22 is stopped.

Because the attachment structure for the second stopper 48 has asymmetric relation to the attachment structure for the first stopper 47,a manufacturing method for the vehicle steering system 1 will bedescribed, mainly focusing on an operation for attaching the firststopper 47.

At first, in a temporarily-fitting step illustrated in FIG. 5A, thefirst stopper 47 is temporarily fitted to the fitting surface 53 suchthat the first stopper 47 is fitted only to the loose fit portion 53 aof the fitting surface 53 of the tubular housing 5 (the first housing37).

A temporarily-fitting step for the second stopper 48 (not shown) issimilar to the temporarily-fitting step for the first stopper 47. Thatis, the second stopper 48 is temporarily fitted to the fitting surface54 such that the second stopper 48 is fitted only to the loose fitportion 54 a of the fitting surface 54 of the tubular housing 5 (thesecond housing 38).

Then, the first and second housings 37, 38 are assembled from therespective sides of the steered shaft 6 so that the state illustrated inFIG. 4 is achieved.

Subsequently, in a fully-fitting step for the first stopper 47, asillustrated in FIGS. 5B to 5C, the ball nut 33 is rotationally driven bythe electric motors 21, 22 to move the steered shaft 6 in the axialdirection X1, so that the first stopper 47 is fully fitted to the tightfit portion 53 b of the fitting surface 53 by the first contact portion45 of the first end portion 321 of the threaded shaft 32.

When the first stopper 47 contacts the receiving portion 55 of thehousing 5, the steered shaft 6 becomes immovable in the axial directionX1. This causes the rotor 26 of the electric motors 21, 22 to benon-rotatable. The fact that the rotor 26 has become non-rotatable isdetected based on an output from the rotation angle sensor 42, and thefully-fitting step is finished by stopping the driving of the electricmotors 21, 22 at the timing that is reached after the predetermined timehas elapsed from the timing of the detection.

More specifically, as illustrated in a flow of FIG. 6, the electricmotors 21, 22 constituting the steering actuator 4 are started to bedriven (step S1). Then, a signal from the rotation angle sensor 42 isreceived so as to monitor whether the rotor 26 has stopped or not, basedon a result of detection of the rotation angle sensor 42 (steps S2, S3).When the fact that the rotor 26 has stopped is detected in step S3 (NOin step S3), counting by a timer is started (step S4). When counting-upby the timer is finished (YES in step S5), the driving of the electricmotors 21, 22 is stopped (step S6), and thus the process is finished.

An operation for attaching the second stopper 48 is similar to theoperation for attaching the first stopper 47.

With the vehicle steering system 1 according to the present embodiment,based on a moving direction of the steered shaft 6, one of the contactportions 45, 46 of the steered shaft 6 passes through the rotor 26 so asto contact a corresponding one of the stopper 47, 48, therebyrestricting a travel amount of the steered shaft 6. Because both thestoppers 47, 48 are constituted by members different from the housing 5,it is possible to form the stoppers 47, 48 from a material having astrength higher than that of the housing 5, thereby making it possibleto improve a strength of the vehicle steering system 1.

Further, the first and second contact portions 45, 46 housed in therotor 26 are provided respectively at the first and second end portions321, 322 of the threaded shaft 32 provided at the intermediate part ofthe steered shaft 6. Accordingly, it is possible to restrict the travelamount of the steered shaft 6 within the housing 5, while reducing thesize in the axial direction X1 and the radial direction. Further, eachof the stoppers 47, 48 do not run on the threaded shaft 32.

Furthermore, because the contact portions 45, 46 are formed from thesame material as that of the steered shaft 6 and formed integrally withthe steered shaft 6, it is possible to simplify the structure. Further,in comparison with a case where the contact portions are constituted bymembers different from the steered shaft, it is possible to obtain thecontact portions 45, 46 having a sufficient strength, while reducing thesize in the axial direction X1 and the radial direction.

Further, the annular stoppers 47, 48 are fitted to the correspondingfitting surfaces 53, 54 of the housing 5, and the stoppers 47, 48 arereceived by the corresponding receiving portions 55, 56 of the housing 5in the axial direction X1. Therefore, assembly is easily performed, and,further, the stoppers 47, 48 are reliably retained.

Further, when the stoppers 47, 48 are fitted to the correspondingfitting surfaces 53, 54, the stoppers 47, 48 are temporarily fitted tothe loose fit portions 53 a, 54 a of the corresponding fitting surfaces53, 54 at first, and then, fully fitted to the tight fit portions 53 b,54 b of the corresponding fitting surfaces 53, 54. Accordingly, afterthe stoppers 47, 48 are temporarily fitted to the corresponding fittingsurfaces 53, 54, the steered shaft 6 is driven axially, thereby allowingthe stoppers 47, 48 to be fully fitted to the corresponding fittingsurfaces 53, 54 by pressing the stoppers 47, 48 in the axial directionX1 by the corresponding contact portions 45, 46.

That is, as shown in the manufacturing method for the vehicle steeringsystem 1 according to the present embodiment illustrated in FIG. 4 andFIGS. 5A to C, after the stoppers 47, 48 are temporarily fitted to theloose fit portions 53 a, 54 a of the fitting surfaces 53, 54 formed atthe respective end portions 51, 52 of the tubular housing 5 (thetemporarily-fitting step illustrated in FIG. 5A), the steered shaft 6 ismoved in the axial direction X1 so that, by the first and second contactportions 45, 46 formed respectively at the first and second end portions321, 322 of the threaded shaft 32 at the intermediate part of thesteered shaft 6, the corresponding stoppers 47, 48 are fully fitted tothe tight fit portions 53 b, 54 b of the corresponding fitting surfaces53, 54 (the fully-fitting step illustrated in FIGS. 5B to C).

Further, in the fully-fitting step, the steered shaft 6 is driven in theaxial direction X1 via the ball screw mechanism 23 by the electricmotors 21, 22 housed in the housing 5, thereby allowing the stoppers 47,48 to be fully fitted to the corresponding fitting surfaces 53, 54.

Further, in the fully-fitting step, it is possible to detect the timingat which the stoppers 47, 48 contact the corresponding receivingportions 55, 56 and the steered shaft 6 becomes immovable in the axialdirection X1, as the timing at which the rotor 26 actually stops, basedon an output from the rotation angle sensor 42 (see steps S2, S3 in FIG.6). Thus, it is possible to finish the fully-fitting step by stoppingthe driving of the electric motors 21, 22 (see step S6 in FIG. 6) at thetiming that is reached after the predetermined time has elapsed from thetiming of the detection (see steps S4, S5 in FIG. 6).

Accordingly, it is possible to avoid the situation where the electricmotors 21, 22 continue to be driven unnecessarily although thefully-fitting step has been finished. As a result, it is possible toprevent damages to the contact portions 45, 46, the stoppers 47, 48, andthe housing 5, and further, it is possible to reliably prevent overloadon the electric motors 21, 22.

In the embodiment in FIGS. 3A, 3B, the end face 45 b serving as acontact surface of the first contact portion 45 contacts the end face 47b serving as a contact surface of the first stopper 47, therebyrestricting the travel amount of the steered shaft 6 in the axialdirection X1. The end faces 45 b, 47 b serving as the contact surfacesare surfaces that are perpendicular to the axial direction X1. Further,the end face 46 b serving as a contact surface of the second contactportion 46 contacts the end face 48 b serving as a contact surface ofthe second stopper 48, thereby restricting the travel amount of thesteered shaft 6 in the axial direction X1. The end faces 45 b, 47 bserving as the contact surfaces are surfaces that are perpendicular tothe axial direction X1.

In contrast to this, FIGS. 7A and 7B illustrate another embodiment ofthe present invention. In the present embodiment, as illustrated in FIG.7A, a conical tapered surface 45 c serving as a contact surface formedat the first contact portion 45 contacts a conical tapered surface 47 cserving as a contact surface formed at the end face 47 b of the firststopper 47, thereby restricting the travel amount of the steered shaft 6in the axial direction X1. An inclination angle of the conical taperedsurface 47 c with respect to the end face 47 b of the first stopper 47is preferably 45° or less, in order to prevent the first stopper 47 fromreceiving an excessive diameter increasing force when the first stopper47 receives the first contact portion 45.

Further, as illustrated in FIG. 7B, a conical tapered surface 46 cserving as a contact surface formed at the second contact portion 46contacts a conical tapered surface 48 c serving as a contact surfaceformed at the end face 48 b of the second stopper 48, therebyrestricting the travel amount of the steered shaft 6 in the axialdirection X1. An inclination angle of the conical tapered surface 48 cwith respect to the end face 48 b of the second stopper 48 is preferably45° or less, in order to prevent the second stopper 48 from receiving anexcessive diameter increasing force when the second stopper 48 receivesthe first contact portion 46.

In the present embodiment, because contact surfaces of the stoppers 47,48 and the corresponding contact portions 45, 46 include the conicaltapered surfaces 47 c, 45 c, 48 c, 46 c, it is possible to ensure alarge pressure receiving area at the time when the respective stoppers47, 48 contact the corresponding contact portions 45, 46 and receive acollision load. As a result, it is possible to have a sufficientstrength even with a small size.

Further, FIGS. 8A, 8B illustrate further another embodiment of thepresent invention. The present embodiment is different from theembodiment in FIGS. 3A, 3B in the following points. That is, asillustrated in FIG. 8A, a guided protrusion 471 protruding in the axialdirection X1 from an end face 47 b of a first stopper 470 is provided,and a guide hole 57 into which the guided protrusion 471 is inserted isformed in a first end portion 51 of the housing 5. Thus, as the guidedprotrusion 471 of the first stopper 470 is inserted into thecorresponding guide hole 57, the first stopper 470 is guided so as to befitted to the fitting surface 53.

Further, as illustrated in FIG. 8B, a guided protrusion 481 protrudingin the axial direction X1 from an end face 48 b of a second stopper 48is provided, and a guide hole 58 into which the guided protrusion 481 isto be inserted is formed in a second end portion 52 of the housing 5.Thus, as the guided protrusion 481 of the second stopper 480 is insertedinto its corresponding guide hole 58, the second stopper 480 is guidedso as to be fitted to the fitting surface 54.

Further, FIG. 9 illustrates yet another embodiment of the presentinvention. The present embodiment is different from the embodiment ofFIG. 4 in the following points. That is, in the embodiment in FIG. 4,the steered shaft 4 is driven in the axial direction X1 by the first andsecond electric motors 21, 22 constituting the steering actuator 4. Inthis way, the fully-fitting step is performed. In contrast to this, inthe present embodiment, the steered shaft 6 is driven in the axialdirection X1 by an actuator 60 provided outside the vehicle steeringsystem 1, without using the steering actuator 4 (the electric motors 21,22). In this way, the fully-fitting step is performed. Note that a loaddetection sensor 61 such as a load cell is provided between the actuator60 and the steered shaft 6, and the steered shaft 6 is driven via theload detection sensor 61.

A control device 190 starts driving of the external actuator 60 asillustrated in a flow in FIG. 10 (step T1). Then, a signal from the loadsensor 61 is received (step T2), and a signal from a rotation anglesensor 42 is received (step T3).

In step T4, it is determined whether or not the rotor 26 has stopped,based on a result of detection of the rotation angle sensor 42. When therotor 26 has stopped (YES in step T4), counting by a timer is started(step T5). When counting-up by the timer is finished (YES in step T6),the driving of the actuator 6 is stopped (step T7), and thus the processis finished.

When a detected load is less than a threshold in step T8 (NO in stepT8), the process returns to step ST2 and repeats steps T2 to T4, and T8.

On the other hand, when it is determined that the rotor 26 has notstopped in step T4 (YES in step T4), the process proceeds to step T8,and it is determined whether or not a load (corresponding to an axialforce of the steered shaft) detected by the load sensor is equal to orhigher than the threshold. When the detected load is equal to or higherthan the threshold (YES in step T8), the process proceeds to step T7 tostop driving of the external actuator 60 immediately and finish theprocess.

In the present embodiment, it is possible to obtain the same effect asthat in the embodiment in FIG. 6. Further, even if the rotor 26 has notstopped actually (i.e., even in the case of NO in step T4 of FIG. 10),it is possible to stop the movement of the steered shaft 6 (i.e., tostop the driving of the actuator as illustrated in step T7 of FIG. 10)based on an axial force of the steered shaft 6 (i.e., based on adetection result in step T8 of FIG. 10). Therefore, it is possible toavoid the situation where external actuator 60 continues to be drivenunnecessarily although the fully-fitting step has been finished. As aresult, an excess load is not applied to the contact portions 45, 46 andthe stoppers 47, 48, and it is possible to prevent damages to thesecomponents and the housing 5. Further, it is possible to reliablyprevent overload on the external actuator 60.

The present invention is not limited to the above-described embodiments.For example, in each of the above-described embodiments, the firststopper 47, 470 and the second stopper 48, 480 are attached respectivelyto the corresponding first and second housings 37, 38 of the housing 5.However, instead of this, as illustrated in FIG. 11, first and secondstoppers 147, 148 may be formed from the same material as that of thefirst and second housings 37, 38 of the housing 5, and formed integrallywith the first and second housings 37, 38, respectively. In this case,it is possible to simplify the structure. In addition, variousmodifications may be made within the scope of appended claims.

DESCRIPTION OF THE REFERENCE SIGNS

1: VEHICLE STEERING SYSTEM, 2: STEERING MEMBER, 3: STEERED WHEEL, 4:STEERING ACTUATOR, 5: HOUSING, 6: STEERED SHAFT, 7: TIE ROD, 8: KNUCKLEARM, 21: FIRST ELECTRIC MOTOR, 22: SECOND ELECTRIC MOTOR, 23: BALL SCREWMECHANISM, 24: FIRST STATOR, 25: SECOND STATOR, 26: ROTOR, 27: ROTORCORE, 27 b: INNER PERIPHERY (OF ROTOR CORE), 32: THREADED SHAFT, 321:FIRST END PORTION (OF THREADED SHAFT), 322: SECOND END PORTION (OFTHREADED SHAFT), 33: BALL NUT, 34: BALL, 37: FIRST HOUSING, 38: SECONDHOUSING, 42: ROTATION ANGLE SENSOR, 45: FIRST CONTACT PORTION, 45 a:OUTER PERIPHERY, 45 b: END FACE (CONTACT SURFACE), 45 c: CONICAL TAPEREDSURFACE (CONTACT SURFACE), 46: SECOND CONTACT PORTION, 46 a: OUTERPERIPHERY, 46 b: END FACE (CONTACT SURFACE), 46 c: CONICAL TAPEREDSURFACE (CONTACT SURFACE), 47, 470, 147: FIRST STOPPER, 47 a: OUTERPERIPHERY, 47 b: END FACE (CONTACT SURFACE), 47 c: CONICAL TAPEREDSURFACE (CONTACT SURFACE), 48, 480, 148: SECOND STOPPER, 48 a: OUTERPERIPHERY, 48 b: END FACE (CONTACT SURFACE), 48 c: CONICAL TAPEREDSURFACE (CONTACT SURFACE), 51: FIRST END PORTION (OF HOUSING), 52:SECOND END PORTION (OF HOUSING), 53: FITTING SURFACE, 53 a: LOOSE FITPORTION, 53 b: TIGHT FIT PORTION, 54 a: LOOSE FIT PORTION, 54 b: TIGHTFIT PORTION, 55, 56: RECEIVING PORTION, 57, 58: GUIDE HOLE, 100:STEERING MECHANISM, 471, 472: GUIDED PROTRUSION, S1, S2: RADIAL GAP, X1:AXIAL DIRECTION

The invention claimed is:
 1. A steer-by-wire vehicle steering systemthat converts rotative power of an electric motor into a movement of asteered shaft in an axial direction via a ball screw mechanism, thevehicle steering system comprising: a threaded shaft provided at anintermediate part of the steered shaft; a ball nut screwed to thethreaded shaft via balls and rotatable together with a rotor of theelectric motor; a tubular housing that houses the electric motor and theball nut and through which the steered shaft is passed; and first andsecond stoppers arranged at first and second end portions of thehousing, respectively, wherein the steered shaft has first and secondcontact portions located respectively at first and second end portionsof the threaded shaft, radial gaps are formed between outer peripheriesof the respective contact portions and an inner periphery of the rotorso that the each of the contact portions is allowed to be inserted intothe rotor, and when the steered shaft moves in the axial direction, oneof an end face of the contact portions, which is on a moving directionside and passed through the rotor, contacts an end face of acorresponding one of the stoppers, so that a travel amount of thesteered shaft in the axial direction is restricted.
 2. The vehiclesteering system according to claim 1, wherein contact surfaces of thestoppers and the corresponding contact portions include conical taperedsurfaces.
 3. The vehicle steering system according to claim 2, wherein:each of the stoppers has an annular shape; and each of the first andsecond end portions of the housing has an annular fitting surface towhich an outer periphery of a corresponding one of the stoppers isfitted, and a receiving portion that receives the corresponding stopperaxially.
 4. The vehicle steering system according to claim 2, whereinthe housing and each of the stoppers are formed of a single material,and formed integrally with each other.
 5. The vehicle steering systemaccording to claim 1, wherein: each of the stoppers has an annularshape; and each of the first and second end portions of the housing hasan annular fitting surface to which an outer periphery of acorresponding one of the stoppers is fitted, and a receiving portionthat receives the corresponding stopper axially.
 6. The vehicle steeringsystem according to claim 5, wherein: each of the first and secondfitting surfaces has a loose fit portion to which a corresponding one ofthe annular stoppers is fitted loosely, and a tight fit portion to whichthe corresponding stopper is fitted tightly; and the tight fit portionis located at a deeper side of a fitting stroke, with which thecorresponding stopper is fitted to the corresponding fitting surface,than the corresponding loose fit portion.
 7. The vehicle steering systemaccording to claim 6, wherein: each of the first and second stoppers hasa guided protrusion extending axially; each of the first and second endportions of the housing has a guide hole into which the guidedprotrusion of a corresponding one of the stoppers is inserted; and asthe guided protrusions of the stoppers are inserted into thecorresponding guide holes, the stoppers are guided so as to be fitted tothe corresponding fitting surfaces.
 8. The vehicle steering systemaccording to claim 1, wherein the housing and each of the stoppers areformed of a single material, and formed integrally with each other.
 9. Asteer-by-wire vehicle steering system that converts rotative power of anelectric motor into a movement of a steered shaft in an axial directionvia a ball screw mechanism, the vehicle steering system comprising: athreaded shaft provided at an intermediate part of the steered shaft; aball nut screwed to the threaded shaft via balls and rotatable togetherwith a rotor of the electric motor; a tubular housing that houses theelectric motor and the ball nut and through which the steered shaft ispassed; and first and second stoppers arranged at first and second endportions of the housing, respectively, wherein the steered shaft hasfirst and second contact portions located respectively at first andsecond end portions of the threaded shaft, radial gaps are formedbetween outer peripheries of the respective contact portions and aninner periphery of the rotor so that the each of the contact portions isallowed to be inserted into the rotor, and when the steered shaft movesin the axial direction, one of the contact portions, which is on amoving direction side and passed through the rotor, contacts acorresponding one of the stoppers, so that a travel amount of thesteered shaft in the axial direction is restricted, wherein: each of thestoppers has an annular shape; and each of the first and second endportions of the housing has an annular fitting surface to which an outerperiphery of a corresponding one of the stoppers is fitted, and areceiving portion that receives the corresponding stopper axially.